The present invention relates to a method and a device for limiting the current supplied to the electrical motor of an electric vehicle which is driven by an electric motor. More particularly, this invention offers a new method for limiting the current supplied to the motor of an electric vehicle to prevent the motor from stopping during a switching operation by thermal switches of control unit or motor, when the motor temperature exceeds a predetermined level; or when motor is being operated in a braking mode, the present invention allows a reduced current to pass through the motor, through as a speed control signal is delivered to the control unit of the motor. According to the method of this invention, the electrical current supplied to the motor is reduced to a low level in multiple steps, when the motor is being overheated or operated in a braking mode.
Conventionally, the motor of an electric vehicle is controlled by a control circuit including a pulse width modulator, a DC/DC converter or an inverter having a switching element, a gate driver and a current limiter for limiting the current supplied to the motor. Generally, the current limiter operates in two ways:
(1) a current detector is provided to feed the motor current back to a controlled loop, and the voltage applied to the motor is adjusted by adjusting the output of the pulse width modulator to the inverter according to the detected feedback current. (2) a current detector is provided to detect the current supplied to the motor, and when the detected current exceeds a predetermined level, an "INHIBIT" signal is produced by the current limiter and sent to the pulse width modulator to cause the inverter to stop the supply of the voltage from the source to the motor, until the current supplied to the motor is reduced to the level lower than a predetermined value, the current limiter then dismisses the "INHIBIT" signal to the pulse width modulator to allow the inverter to resume the supply of voltage to the motor.
Generally, the control unit and motor include respectively a thermal switch which is activated, when the temperature exceeds a predetermined level, to cut off the supply of current to the motor to stop the operation of the motor. However, such a sudden stop of the motor is undesirable when the vehicle is on the road. Furthermore, the driving circuit of a conventional electrically driven vehicle is caused to become "disable" when the brake is applied, and therefore it does not provide adequate control at low speeds. The conventional methods also tend to allow the vehicle to slide down the slope when restarting the motor uphill from a stop. The vehicle will be better controlled if the supply of the current is maintained during the braking operation.